1. Field of the Invention
The present invention relates to a pigment composition preferably used in color filters, inkjet inks and the like. Furthermore, the invention relates to an ink composition that uses the pigment composition and is suitably used for inkjet recording, a printed article obtained by use of the ink composition, and an inkjet recording method that uses the ink composition. In more detail, the invention relates to an ink composition that is excellent in the dispersibility of a coloring agent, excellent in color forming property, cured by irradiation with an actinic radiation ray without generating a volatile component, capable of forming high quality images and suitable for inkjet recording, a printed article obtained by use of the ink composition, and an inkjet recording method that uses the ink composition. Furthermore, the invention relates to a novel polyallylamine derivative suitably usable in the pigment composition and ink composition.
2. Related Art
Electrophotographic systems, sublimation-type and melting-type thermal transfer systems, and inkjet systems are image recording methods of forming an image on a recording medium such as paper based on image data signals. In particular, inkjet systems are applicable to an inexpensive apparatus, and perform direct image formation on a recording medium by ejecting ink only to image areas where ink deposition is necessary; therefore, inkjet systems use ink effectively, thus reducing the running cost. Further, inkjet systems generate less noise and are superior image recording systems.
Inkjet systems enable printing on recording media that do not absorb water, such as plastic sheets and metallic plates, as well as on paper. In inkjet systems, it is an important task to achieve higher printing speed and higher printing quality. Therefore, the time liquid droplets take to dry and cure after printing has significant influence on the sharpness of the image.
In one inkjet system, an inkjet recording ink that can be cured by irradiation with an actinic radiation ray is used (for example, see Japanese Patent Application Laid-Open (JP-A) No. 5-214280). According to this system, sharp images can be formed by curing ink droplets by irradiation with an actinic radiation ray just after printing.
In order to form highly accurate images with excellent color forming properties, such a curable ink composition as a curable ink for inkjet recording is required to have high pigment dispersibility, and stability over time. Reduction in the pigment size is necessary for imparting clear color tone and high tinting strength to the ink composition. In particular, concerning the ink used for inkjet recording, ejected droplets of the inkjet ink exert a significant influence on the sharpness of images; therefore, the ejection quantity of the ink droplets needs to be small, and the size of the pigment particles contained in the ink has to be smaller than the thickness of the film formed by curing of the ink. However, when the size of the pigment particles is reduced so as to achieve higher tinting strength, the fine pigment particles are hard to disperse, and aggregates are easily formed. Another problem is created in that the viscosity of the ink composition is increased by the addition of a dispersant. The formation of the pigment aggregates and the increase in the viscosity of the ink composition both adversely affect ink ejection property, and such an ink composition is not preferred.
When an ink composition is used for inkjet recording, the ink composition is contained in a cartridge. The ink composition in the cartridge is heated at ejection and cooled at a non-ejection time or during storage; in this way, the ink composition undergoes repeated temperature changes (heating-cooling). These temperature changes also adversely affect the pigment dispersibility, and the pigment dispersibility is deteriorated with time, whereby problems are created in that aggregation of dispersed pigments and increase in the viscosity of the ink composition easily occur.
Accordingly, there is a demand for an ink composition having sufficient fluidity and excellent stability of the pigment dispersion over time, in which fine pigment particles are dispersed stably. Various proposals have been made concerning dispersants for achieving stable pigment dispersions.
For improving affinity with the pigment, an ink composition using a pigment derivative as a dispersant (see, for example, JP-A Nos. 2003-119414 and 2004-18656), an ink composition that uses a polymer having a basic group as a dispersant for a specific pigment such as a phthalocyanine-based or quinacridone-based pigment (see, for example, JP-A No. 2003-321628), and an ink composition (see, for example, JP-A No. 2004-131589) which is free of organic solvent and contains a dispersant, such as a poly(ethyleneimine)-poly(12-hydroxystearic acid) graft polymer, and a specific monomer that dissolves the dispersant have been proposed.
In these ink compositions, the pigment dispersibility is certainly improved owing to the function of the dispersant. However, the size of the pigment used in the ink composition is not sufficiently small, and there is a room for improvement of dispersibility of finer pigment particles. Further, the ink composition also has a problem in that the dispersion stability upon long-term storage or upon repeated temperature changes is still insufficient.